HW-9 diode mod / Dayton


From: Bruce Walker (bruce@Think.COM)
Date: Tue Apr 20 1993 - 13:55:27 EDT

First of all, I'm headed to Dayton after Wednesday, so anything sent to
QRP-Request@Think.COM during that time will be serviced next week.

Last night, I put the diode mod into the HW-9 I've been working on. This
mod appeared in one of the magazine articles (QST or 73) on HW-9 mods, but
unfortunately I can't remember which one.

The HW-9 uses diode steering between TX and RX paths for QSK. There are 5
diodes directly in the RF RX path, and Heath used pretty lossy diodes in
the design. The article suggests replacing them with Shottky diodes. I
didn't have any Shottkys, but I did have some cheap Germanium diodes (about
which I know nothing) from Radio Shack. Actually, I have Shottkys, but
they are big (for power applications).

Using 12V and a ~10k resistor in series, I measured the voltage drop across
the Heath parts (.79V!), the big Shottkys (.22V) and the Germaniums (.27V).
Sounds like it's in the right direction; I don't know about the noise of
these components. I put the Germaniums in, and an S6 reference signal on
10m went to S9+20 on the meter. Now, I can't infer much about db from the
meter (this meter seems to take a lot to get it off zero, and very little
to get it to S9 once it's moving), but I'll call it a qualitative victory.

The same article suggests replacing the first mixer (a dual-gate MOSFET)
with something which doesn't fall off as much over the HF range...both
these mods are to to liven up the RX performance on the higher frequencies.
I'll probably try that after Dayton.

I also finally found a cold solder joint which explained the flakey AGC
performance I had been having.

72 --bruce WT1M

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